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Last winter, the U.S. Preventive Services Task Force (USPSTF) gave a positive recommendation (B grade) for lung cancer screening with low-dose computed tomography. The recommendation applies to high-risk adults aged 55 to 80 years with a smoking history of at least 30 pack-years who are still smoking or have quit within the past 15 years (1). The USPSTF based its recommendation on the landmark National Lung Screening Trial (NLST) (2), which provided evidence about the efficacy of screening but did not answer questions about effectiveness when implemented in everyday clinical practice. The recommendation, as noted in an accompanying editorial, leaves many of these critical issues unaddressed (3). Indeed, the Medicare Evidence Development & Coverage Advisory Committee recently concluded that there was inadequate evidence to recommend national coverage for lung cancer screening (4). This conclusion highlights the controversy over what constitutes sufficient evidence to support widespread implementation of a new technology. It has also heightened the tension between advocates who are eager to see wide deployment of screening quickly and others who are concerned about the ability of health care systems to provide a low-dose computed tomography screening program to the potentially large number of patients meeting criteria while still minimizing harms (4). That a new intervention can work in randomized trials does not guarantee that it will work as well when delivered in real-world practice, nor that the benefits in the community will justify the possible harms and costs (including opportunity costs). Should these unanswered questions delay implementation of screening pending further study, or do the potential benefits compel their early adoption?